I sculpted a plain, but I can't texture paint it
Texture Painting in Cycles is a bit different. You cannot just go in texture paint mode and start painting it. To start texture painting,


Select the object you want to paint on and unwrap it.
Give it a new material. Make it diffuse.

Go to Texture Paint mode and create a new paint slot. Change it to 'Diffuse Color'. In the pop-up menu, change it's dimensions to the dimensions of the texture you want and select the base color. Click OK.

Start Painting!


This youtube tutorial might also help you.